Anthrax lethal toxin comprises two proteins: protective antigen (PA; MW 83 kDa) and lethal factor (LF; MW 87 kDa). We have recently determined the crystal structure of the 735-residue PA in its monomeric and heptameric forms (Petosa et al. 1997). It bears no resemblance to other bacterial toxins of known three-dimensional structure, and defines a new structural class which includes homologous toxins from other Gram-positive bacteria. We have proposed a model of membrane insertion in which the water-soluble heptamer undergoes a substantial pH-induced conformational change involving the creation of a 14-stranded beta-barrel. Recent work by Collier's group (Benson et al. 1998) lends strong support to our model of membrane insertion. 'Lethal factor' is the catalytic component of anthrax lethal toxin. It binds to the surface of the cell-bound PA heptamer and, following endocytosis and acidification of the endosome, translocates to the cytosol. We have made substantial progress towards an atomic resolution crystal structure of LF. Progress towards a structure of the 7:7 translocation complex between the PA heptamer and LF will also be discussed.